BUG #14363: CASE against a literal value produces correct against, same expression against a column null doesn't

The following bug has been logged on the website:

Bug reference: 14363
Logged by: Henry Cate
Email address: hcate3(at)gmail(dot)com
PostgreSQL version: 9.5.2
Operating system: x86_64-pc-linux-gnu
Description:

With this setup:

drop table if exists t2;
create table t2 (
order_key int,
decimal_9_1 decimal(9,1),
decimal_18_1 decimal(18,1));

insert into t2 values (3, 901.8, null);

This query:

select decimal_9_1, decimal_18_1, (
case decimal_9_1
when null then 0
when decimal_9_1 then 1 end),
case (
case decimal_9_1
when null then 0
when decimal_9_1 then 1 end)
when 1 then 2
when 0 then 3
end,
case (
case decimal_9_1
when decimal_18_1 then 0
when decimal_9_1 then 1 end)
when 1 then 2
when 0 then 3
end
from t2 ;

produces these results:

decimal_9_1 | decimal_18_1 | case | case | case
-------------+--------------+------+------+--------
901.8 | <null> | 1 | 2 | <null>
(1 row)

I expect the last two columns to both have a value of 2.

The fourth column compares the result of the inner case statement to NULL
and produces the correct result. The last column compares to a column which
does have NULL, but some how Postgres gets confused and returns NULL. It
should also be returning 2.

Here is the version information:

ybd_test=# select version();
version

--------------------------------------------------------------------------------------------------------------------
PostgreSQL 9.5.2 on x86_64-pc-linux-gnu, compiled by clang version
3.8.0-2ubuntu3 (tags/RELEASE_380/final), 64-bit
(1 row)